This invention relates to an improvement in a process for manufacturing the material for ceramic cutting tools, and more specifically, to the powder mixture which is used in the process, and to the ceramic cutting tool materials prepared according to this process.
Properly speaking, in an evaluation of the properties of ceramic cutting tool materials, the following physical and mechanical properties are necessary.
Those properties are superior hardness with sufficient toughness at ordinary temperatures which is retained even during the superheated state of the cutting operation thereby resulting in superior resistance to abrasion and wear, and greater mechanical strength such as bending rupture strength and compressive strength which will minimize chipping or fractures. Furthermore, less affinity between cutting tool material and the metal to be cut is also required.
Other than the above requirements, one of more importance for ceramic cutting tool materials is thermal shock resistance which will minimize heat cracks caused by sudden heating and cooling. This property is especially necessary to eliminate failures due to the cracks which might arise by repeated heating and cooling of the edge of a knife blade during an intermittent cutting operation.
For the purpose of improving the various properties described above, a procedure employing titanium carbide as a constituent incorporated in a powder mixture consisting principally of alumina which is sintered by hot-pressing has been devised heretofore. A feature of an embodiment of the previous process which comprises mixing 15-80% of powdered titanium carbide by weight with powdered alumina, incorporating the thus produced mixture in a graphite mold and sintering by hot-pressing at pressures of about 400kg/cm.sup.2 and temperatures of 1500.degree.-1800.degree. C has been advocated.
However, to some extent this method necessitates employing alumina of high purity as well as very fine particle size which has an improved sinterability as compared to that of the prior art. Therefore it was very hard to obtain a sintered body having a high density when using commercial alumina as the raw material in an ordinary process.
Furthermore, since sinterability of the above described powdered mixture is also dependent upon the purity of powdered titanium carbides, obtaining a sintered body with a high relative density in excess of 98.5% was difficult unless-optimal conditions were achieved for each operation during hot-pressing.
The reason why the sintered body should have a relative density greater than 98.5% is due to the fact that if the body has a density lower than the above percent, the cutting performance of the cutting tools made therefrom is extraordinarily low.
The present invention has been achieved with the intention of overcoming these defects described above and has not been contemplated or even romotely suggested by the prior art.
It is an object of the present invention to provide a process for producing material for manufacturing cutting tools which have improved physical and mechanical properties as compared to those of the prior art.
It is another object of the present invention to provide a powder mixture which is used in the manufacturing of the material described above and which has improved sinterability.
It is still a further object of the present invention to provide novel ceramic cutting tool materials which have improved hardness and mechanical strength as compared to the prior art.